Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats

"bulb. At the time of our observations in 1909 and 1910 there was an area of nearly-flat ice, merging into an area of moderately-rising glacier which sloped gradually upwards to the valley mouth. Upon the edge of this flat ice a broad stream was slowly flowing through a series of lake-like expanses in which clay and sand were being deposited upon the ice, as they had previously been deposited upon a similar but higher &rea to the northeast where slumping had followed the melting. A continuation of such alluviation upon the ice will produce a gravel-bottomed interior flat, similar to that of Variegated Glacier in Yakutat Bay. The ice surface in the flat, which was diversified only by broad swells and sags, had melted down to the bottoms of the crevasses. There was an occasional one in 1910 which had sloping walls and was 4 or 5 feet deep. Farther up the flat toward the valley glacier the crevasses were deeper but SO feet was the maximum depth. Some of the shallower crevasses were filled with sand and mud. There were no appreciable changes in the ice or the drainage of the interior flat between 1909 and 1910.
The Northern Terminal Moraine, North of the bulb of Allen Glacier is a strong cres-centic terminal moraine (PI. CLXXIE). It is over 3 miles long, an eighth to a quarter mile wide, and 50 feet high. Parts of it form a single broad ridge, other parts have shallow basins and flat knobs. It is separated from the glacier by a crescentic lowland a quarter to a half mile wide with pools, marginal channels, outwash deposits and lake clays, and slumping surface where these deposits had been laid down upon the nose of the glacier. This zone is barren of all vegetation.
The moraine seems to have no ice beneath it. Its surface is covered with moss and grasses and had alders and willows which were 9 to 19 years of age in 1910. One willow at the outer border of the moraine on the alluvial fan was approximately 50 years old.
The only gaps in this northern moraine were where streams from the glacier had cut through and only one of these was occupied in 1910. The other gaps were abandoned stream channels which headed up in the moraine or cut completely through it and terminated at a higher level than the present surface of the glacier.
Alluvial Fans of Outwash Gravel. The excess in the amount of thinning of the glacier by ablation in the unprotected interior flat has given the northern side a lower position than the moraine-protected southern margin and the main drainage from Allen Glacier, therefore, escapes this way, building the great alluvial fan, many times larger than that of the stream from the southern side. This northern fan (PI. CLXXFV) slopes 54 feet to the mile in one portion and elsewhere 80 feet to the mile. It partly dams Copper River, aiding the glacier bulb in producing the lake-like expanse which backs up into side valleys and produces mud flats at low water and broad shallow expanses at times of flood (PL CLXXIX). The size of the alluvial fan on the north furnishes additional proof that the glacier has been where it now is for a long period; and the absence of vegetation on all parts of the fan shows that the streams have shifted back and forth over it regularly in building it up.
The Marginal Channel. The marginal channel of Baird Canyon (PI. CLXXI) differs from that at Abercrombie Rapids in the lack of swift, white water with reefs and dangers to navigation, partly because the river is here in a broader channel and partly because the lake-like expanse above Abercrombie Rapids backs up into Baird Canyon and decreases the velocity there. On this account, and also because of the protection afforded by the large alluvial fan on the northern side of the glacier the river is not